If you need to go to a bricks & mortar UK seller, John Lewis are hard to beat, plus you can browse online first.
If you can order online and have a UK seller deliver to Spain, or have the wherewithal to receive deliveries in the UK, then I have had uniformly good experiences with Lambdatek and ebuyer, also cleverboxes, but that was for server stuff.
If buying a ready-made PC or laptop, you might as well get Win 7, as there's little difference in price (if any) between OS and no-OS. Win 8.1 only might be a deal-breaker, unless you are prepared to fork out separately for Win7 (check drivers) or just install Linux.
SD